Gland seal water is water pumped into a cavity beside the pump seal. The gland seal water both leaks out the seal and leaks into the fluid being pumped.
It's used for many reasons.
You'll find it on condensate pumps to prevent air from entering the condenser when the condensate pump is idle. (Typically the seal is under pressure when the pump is running).
On feed pumps, heater drain pumps, and rector coolant pumps it is used to prevent the water from flashing to steam once leaving the pump because the water being pumped is above 100°C. If the water were to flash in the seal, the seal would rapidly erode by the two phase flow.
In the case where the pump is handling radioactive water, it also keeps the radioactive water from escaping. At a PWR, the primary loop seal water comes from the reactor makeup and the condensate, and feed pumps comes from the output of the condensate pumps. I'm not familiar with BWR systems.